1. Field of the Invention
The present invention relates to filters employed in optical communication systems, and more particularly, to dielectric multi-layer interference filters for optical communication systems.
2. Background Art
Multiple-port, filtering and isolating packages are widely used in local and long distance optical telecommunication networks. These networks comprise various spectral shaping and isolating optical filter assemblies as parts of dense wavelength division multiplexing (DWDM) systems. There is an important need to design reliable filters for such systems, which are subject to various thermal and mechanical loads during their 20 to 25 year lifetime. A typical filter manufacturing process consists of polishing a glass substrate to a specific thickness, cleaning it, placing it into a dielectric substrate coating chamber, applying layers of various materials via an e-beam or ion beam assisted sputtering process or equivalent, removing it from the chamber, cutting or dicing to a desired size, then individually measuring to select the diced filters according to desired optical performance. The number of layers applied during the coating process varies depending on the desired optical performance. Layers are xe2x80x9cstackedxe2x80x9d or alternated based on the type of material being used. Each layer has different expansion qualities relative to the neighboring/contiguous layers. These differences, coupled with the environment in the coating chamber, yields a significant amount of filter stress, i.e., the individual layers expand or contract differently than the adjacent layer(s). This becomes an issue when the substrate is diced for final filter size requirements. The filter stress across the round coating substrates is altered significantly when small square filters are cut from them. By introducing sharp corners, the filter stress is effectively focused in the corners.
This stress focusing changes the optical performance of the filter element. The net result is that the usable filter region is reduced. This is a significant problem when trying to minimize filter size and package the filters in an optical device with a collimated light beam size that needs to be located in the correct area of the filter. The light beams and a majority of elements in common filter packages have a circular cross section. Holding the square filter in a round package offers two specific challenges: (1) Packaging the filter without inducing more stress to the films or substrates; (2) locating the optimal location for the collimated light beam.
There are two different technical solutions used in the design of bonds securing the components of a filter assembly. A low compliance bond between thermally well-matched glass components is an approach commonly used by a majority of manufacturers. The adhesives used are heat-curable epoxies with high Young""s modulus (E greater than 10,000 psi) and moderate to high thermal expansion coefficients (xcex1=40 to 60 10xe2x88x926xc2x0 C.xe2x88x921). A typical example is 353ND Epo-Tek epoxy adhesive. In addition, the bond thickness used is very small.
Silicon adhesives are used to bond thermally mismatched elements and glass filters with metal holders. In these joints, a high compliance design is used. The silicones, which can be cured between 20-150xc2x0 C. in the presence of moisture, are typically characterized by an extremely low Young""s Modulus (E less than 500 psi) and high thermal expansion (xcex1=180 to 250 10xe2x88x926xc2x0 C.xe2x88x921). A typical example is DC 577 silicone, which can be used to bond a metal filter holder to a filter.
Adhesive bonding with subsequent soldering or welding is required to encapsulate a filtering assembly into a three-port package or DWDM device. A precise alignment achieved during initial assembly of a filter prior to final packaging, can be easily diminished or ruined due to the high temperature thermal cycle associated with soldering or welding during packaging of the component. Such prior art manufacturing processes and resulting components have several problems resulting from the fact that the optical components experience stresses due to the thermal contraction mismatch between the glass and metal materials; polymerization shrinkage in adhesive bonds; and structural constraints induced by bonding and final soldering during encapsulation. These stresses lead to displacements of optical components during bonding and soldering, resulting in 0.3 to 1.0 dB increase in insertion loss.
Such a filter package enclosure, which is typically formed of six to eight concentric proactive units, has micron transverse tolerances. Maintaining these tolerances requires precision machining, time-consuming alignment, and soldering with frequent rework. As a result of these limitations, the optical performance specifications are lowered and cost is increased. As an example, soldering typically includes several re-flow cycles. This induces local thermal stresses in the nearby adhesive bonds and leads to the degradation of the polymer adhesive, resulting in repositioning of optical components and a shift in the spectral filter performance. With such designs, soldering may also result in the contamination of optical components through direct contact with molten solder and/or flux.
Although the collimating subassemblies and housings are cylinders, the alignment of commercially available optical components, which exhibit a random distribution of optical and structural characteristics, requires some lateral and angular repositioning of the subassemblies. This repositioning of the optical subassemblies is limited by the gap in the solder joint and the ratio of this gap to the length of the subassembly. The lateral and angular repositioning observed in some isolators can be as high as 0.05 to 0.3 mm and 0.5 to 1.5xc2x0 respectively. The soldering of non-capillary gaps produces well-known difficulties, such as high volume shrinkage of the solder, void formation, and contamination of optical components.
However, for many applications, it is desirable to obtain a high accuracy thermally compensated filtering or isolating package that can be relatively inexpensive and reliable. Additionally, a package design should be adequate not only to mechanically protect the fragile optical components, but also to compensate for and minimize the thermally induced shift in spectral performance. Thus, there exists a need for a process for manufacturing a filter (or isolating) element which is miniaturized, has a low insertion loss, is inexpensive to manufacture, which results in a filter having low stress effects, and is reliable, and yields long-term operation.
The present invention comprises an optical filtering element with minimized edge effects, larger clear center aperture, and thermally stable bonding area. The element consists of a modified ball lens. The glass material that comprises the ball lens can be selected from a variety of thermally matched glass specific to the intended application such as BK7, SiO2, et cetera. The lens is polished on two sides so that the polished surfaces are nearly parallel. The polished ball lens(es) are then loaded into a coating fixture and then into a coating chamber. Multiple dielectric layers are applied to one of the polished surfaces, creating a spectral modifying filter. The other side is coated with an anti-reflection coating (some applications do not require the anti-reflection coating). At the completion of the coating run, the now polished and coated ball lenses can be spectrally measured and binned for use.
The polished ball lens filter is a significant improvement over the current square filter. There are no dicing (cutting) induced edge effects. This allows for a more thermally stable filter, avoids chipping of corners due to dicing, and maximizes the usable filter area in a cylindrical package. The outer diameter of the polished and coated ball lens provides a circular, smooth surface that can be used for bonding and alignment, with no risk of epoxy in the optical path. Additionally the spherical shape of these filter elements allows for angular alignment of the filter surface with no change in bond line configuration. Because of this, adhesives that would normally be rejected for use with the square filters can be considered for use with the spherically shaped filter because the bond lines will remain uniform through any angular alignments.